The performance of magnetic heads is dominated by the accuracy of a gap formed therein. A gap in a small size or miniature magnetic head has a very small length of the order of 1 to 2 microns, and extreme difficulties are experienced in forming such a gap with a high accuracy. In the prior art practice, a gap of the kind described has been formed by disposing a pair of core block pieces in abutting relationship so that their gap defining surfaces are located opposite to each other, with a thin sheet of mica or metal foil interposed therebetween as a spacer. Subsequently molten ceramic or like material is caused to permeate into the clearance thus formed, and then allowed to be solidify. However, the spacer material used in this technique is expensive in itself and must be worked to a thickness of 1 to 2 microns with a high accuracy, resulting in an overall cost increase for the manufacture of cores.